The conventional method for extracting copper from its more common natural form, the sulphide ore, involves roasting of such sulphides, producing sulfur dioxide, a gas, and the metal oxide.
This oxide is later reduced with any adequate reagent, mainly carbon, to a solid metallic form, that finally is refined electrochemically to the degree of purity required in the diverse destiny of the metal.
This conventional procedure, centuries old in its main features, has serious disadvantages today due to four main reasons: (a) Air quality laws, limiting drastically the SO.sub.2 emission to the atmosphere, (b) increasing energy cost, (c) decreasing quality of copper deposits, and (d) the requirement of building high capacity plants to meet competitive costs.
These problems have promoted new interest in the old idea of processing the sulphides through a chloride route, instead of the oxide route.
This idea avoids air pollution, because its main leaching reaction, EQU MeS+Cl.sub.2 .fwdarw.Me Cl.sub.2 +S.degree.
leaves the sulfur content of the sulphides in elemental form, easily disposable or marketable as a solid.
This idea requires electrolysis of the produced metal chlorides, and reclaiming the chlorine. Otherwise, the chlorine cost will adversely affect the economics of the process.
The electrolysis of copper chloride has three main requirements: (1) separate chambers for extraction of anodic and cathodic products, in order to avoid toxic effects of chlorine in the working area; (2) maximum current density and compactness on the cell, to minimize the investment costs; and (3) a high purity catholyte, to prevent deposition of impurities along with the copper.
Conventional copper sulphate electrolysis is carried out at 200 A m.sup.-2 to prevent voltage build up and deposition of impurities. When current density rises above that value, the resulting copper cathode fails in meeting quality standards.
Copper chloride electrolysis has been attempted, to various degrees of success, by several groups. The CYMET process (1), the work by the Bureau of Mines (2), Elken (3) and Clear-Duval (4) are the better known ones. They carry out the electrolysis in a molten salt electrolyte, or an aqueous solution of the salt, using a diaphragm to separate the anolyte and catholyte.
(1) U.S. Pat. No. 3,673,061 PA1 (2) Complex Sulphide ores Symposium. Rome October 1980 p. 135-140 PA1 (3) Ibid. p. 135-140 PA1 (4) U.S. Pat. No. 3,879,272
The process using molten salt electrolysis requires purification of the salt feed. It is accomplished via crystallization. On the whole, the energy consumption, and the cumbersome handling of high temperature solid and molten streams, have discouraged any potential users.
Diaphragm cells for electrolysis of dissolved salts suffer from low current efficiency and poor copper quality, due to the presence of impurities in the electrolyte.